I'm certainly no expert on this, but will share my recent experience on the subject. I also have a 1987 300E (98,700 miles), purchased 12/05. I had a refrigerant leak at the compressor (which appeared to be original), and the system had been retrofitted to 134a in 2001 by previous (original) owner. I put on a new (rebuilt) compressor with 6.5 ounces of mineral oil and a new dryer, evacuated the system for a period of about 14 hours, then let the pressure hold for 2 hours with the vacuum pump off. Having no apparent leak, I put in three 6-ounce cans of Industrial 12a refrigerant. The sight glass was clear, and the inside temperature was quite COLD. Colder even than R12, six ounces of this stuff is equivalent to 18 ounces of R12, or to 16 ounces of 134a, and it is compatable with either. Made for automotive use, it is also compatable with all refrigerant oils, but mineral oil is best, as it absorbs less moisture. No license required for this product. Made in USA and totally environmentally-friendly.

Not selling the stuff, just sharing some information on a popular subject.